In construction machines such as hydraulic excavators and the like, the mainstream is a work machine using a hydraulic circuit so called open circuit wherein hydraulic fluid is fed from a hydraulic pump to a hydraulic cylinder through a throttle configured by a control valve while the hydraulic fluid (return hydraulic fluid) flowing out from the hydraulic cylinder is discharged into a hydraulic fluid reservoir. The hydraulic circuit called an open circuit uses the throttle configured by the control valve and hence, is large in pressure loss attributed to the throttle.
In recent years, there have been developed work machines using a hydraulic circuit so called closed circuit in which connections are made annularly (in a closed circuit style) so that hydraulic fluid discharged from a hydraulic pump/motor is fed directly to a hydraulic actuator such as a hydraulic cylinder, a hydraulic motor or the like while the hydraulic fluid upon completion of a prescribed work by driving the hydraulic actuator is returned directly to the hydraulic pump/motor. The hydraulic circuit called a closed circuit is excellent in the performance of fuel efficiency because of being little in pressure loss attributed to the throttle and enabling the hydraulic pump/motor to regenerate the energy owned by the return hydraulic fluid from the hydraulic actuator. Further, there has also been proposed a hydraulic circuit wherein these closed circuit and open circuit are combined.
A swing deceleration regenerative control has been known as one of control techniques for hydraulic circuits called closed circuits of this kind. The swing deceleration regenerative control is designed so that during a swing deceleration of a revolving upperstructure in a work machine, the hydraulic pressure force (brake force) resistant to inertia energy (hereafter referred to as “swing deceleration regenerative energy”) makes a hydraulic pump/motor that is connected to a hydraulic pump in a closed circuit style, operate as a hydraulic motor, so as to assist the driving of an engine or the like and hence, to reduce fuel consumption. That is, the force generated by the driving of the hydraulic pump/motor is transmitted to a drive source such as an engine or the like through a power transmission mechanism such as gears, so that the energy that is originally required for the driving of the drive source can be reduced. In particular, where the drive source is an engine, it becomes possible to reduce the consumption of light oil required to drive the engine. Like this, by the use of the swing deceleration regenerative control, the reduction of fuel consumption becomes possible.
Further, Patent Literature 1 discloses prior art in which closed circuits of this kind are combined. In Patent Literature 1, a plurality of closed circuits are provided in each of which one hydraulic pump/motor is independently connected to each of a plurality of hydraulic actuators such as hydraulic cylinders, hydraulic motors and the like so that the operation speed of each hydraulic actuator is controlled by the control of the discharge flow rate of the hydraulic fluid from each hydraulic pump/motor. Further, the hydraulic circuit is provided therein with a flow path that is for merging the hydraulic fluids discharged from two hydraulic pumps/motors connected to a plurality, for example two, of closed circuits, and the flow path is provided with a flow combining valve. At a high-speed driving of the hydraulic actuator, the flow combining valve is operated to open, so that the hydraulic fluids discharged from these two hydraulic pumps/motors are merged to be supplied to the hydraulic actuator.